Keep throwing dyno at those little craters on the floor when he gets there. All u gotta do is keep a good distance get an aim at the crater time it till he gets there and throw. Keep doing it, it took me hours to figure out and its fucking EASY as shit. You also might have to go do some extra missions to get ur stats up if u aint high enough.

yeah it took me about 30 minutes to figure out how to do it. once i figured it out it was mad easy.

It's not really hard, but you just need to know how. First, he'll run around
shooting at you, and throwing dynamite. Use your dynamite bow, and keep your
distance. Notice the blue/green-looking things on the floor? Time a dynamite
arrow, so it'll explode when Magruder walks over it. Once you did this enough
times, he'll get angry and move onto a platform.

First pull out your sniper, and aim at his head. When you shot him there,
he'll throw a big group of dynamite. Shoot that when it's in mid-air. You need
to shoot it a few times in mid-air, to crush him under a big rock. When you
run out of ammo for your sniper, move closed and use quick-draw to aim at his
head. One shot in the head, and he'll throw the dynamite.

Note that when you move too far away, some henchmen of Magruder will join the
party, so avoid that.

Watch the ending and credits, and pad yourself on the back. Another job well
done.